FIGS. 1A and 1B show the construction of a conventional microwave plasma processing apparatus 100 having such a radial line slot antenna. Where, FIG. 1A shows the microwave plasmas processing apparatus in a cross-sectional view, while FIG. 1B shows the construction of the radial line slot antenna.
Referring to FIG. 1A, the microwave plasma processing apparatus 100 has a processing chamber 101 evacuated from plural evacuation ports 116, and there is formed a stage 115 for holding a substrate 114 to be processed. In order to realize uniform processing in the processing chamber 101, a ring-shaped space 101A is formed around the stage 115, and the plural evacuation ports 116 are formed in communication with the foregoing space 101A with a uniform interval, and hence in axial symmetry with regard to the substrate. Thereby, it becomes possible to evacuate the processing chamber 101 uniformly through the space 101A and the evacuation ports 116.
On the processing chamber 101, there is formed a plate 103 of plate-like form at the location corresponding to the substrate 114 on the stage 115 as a part of the outer wall of the processing chamber 101 via a seal ring 109, wherein the shower plate 103 is formed of a dielectric material of small loss and includes a large number of apertures 107. Further, a cover plate 102 also of a dielectric material of small loss is provided on the outer side of the shower plate 103 via another seal ring 108.
The shower plate 103 is formed with a passage 104 of a plasma gas on the top surface thereof, and each of the plural apertures 107 are formed in communication with the foregoing plasma gas passage 104. Further, there is formed a plasma gas supply passage 106 in the interior of the shower plate 103 in communication with a plasma gas supply port 105 provided on the outer wall of the processing vessel 101. Thus, the plasma gas of Ar, Kr or the like supplied to the foregoing plasma gas supply port 105 is supplied to the foregoing apertures 107 from the supply passage 106 via the passage 104 and is released into a space 103B right underneath the shower plate 103 in the processing vessel 101 from the apertures 107 with substantially uniform concentration.
On the processing vessel 101, there is provided a radial line slot antenna 110 having a radiation surface shown in FIG. 1B on the outer side of the cover plate 102 with a separation of 4–5 mm from the cover plate 102. The radial line slot antenna 110 is connected to an external microwave source (not shown) via a coaxial waveguide 110A and causes excitation of the plasma gas released into the space 101B by the microwave from the microwave source. It should be noted that the gap between the cover plate 102 and the radiation surface of the radial line slot antenna 110 is filled with the air.
The radial line slot antenna 110 is formed of a flat disk-like antenna body 110B connected to an outer waveguide of the coaxial waveguide 110A and a radiation plate 110C is provided on the mouth of the antenna body 110B, wherein the radiation plate 110C is formed with a number of slots 110a and slots 110b wherein slots 110b are formed in a direction crossing the slots 110a perpendicularly as represented in FIG. 1B. Further, a wave retardation plate 110D of a dielectric film of uniform thickness is inserted between the antenna body 110B and the radiation plate 11C.
In the radial line slot antenna 110 of such a construction, the microwave supplied from the coaxial waveguide 110 spreads between the disk-like antenna body 110B and the radiation plate 110C as it is propagated in the radial direction, wherein there occurs a compression of wavelength as a result of the action of the wave retardation plate 110D. Thus, by forming the slots 110a and 110b in concentric relationship in correspondence to the wavelength of the radially propagating microwave so as to cross perpendicularly with each other, it becomes possible to emit a plane wave having a circular polarization state in a direction substantially perpendicular to the radiation plate 110C.
By using such a radial line slot antenna 110, uniform plasma is formed in the space 101B right underneath the shower plate 103. The high-density plasma thus formed is characterized by a low electron temperature and thus, there is caused no damaging of the substrate 114 and there is caused no metal contamination as a result of the sputtering of the vessel wall of the processing vessel 101.
In the plasma processing apparatus of FIG. 1, it should further be noted that there is provided a conductive structure 111 in the processing vessel 101 between the shower plate 103 and the substrate 114, wherein the conductive structure is formed with a number of nozzles 113 supplied with a processing gas from an external processing gas source (not shown) via a processing gas passage 112 formed in the processing vessel 101, and each of the nozzles 113 releases the processing gas supplied thereto into a space 101C between the conductive structure 111 and the substrate 114. It should be noted that the conductive structure 111 is formed with openings between adjacent nozzles 113 with a size such that the plasma formed in the space 101B passes efficiently from the space 101B to the space 101C by way of diffusion.
Thus, in the case a processing gas is released into the space 101C from the conductive structure 111 via the nozzles 113, the processing gas is excited by the high-density plasma formed in the space 101B and a uniform plasma processing is conducted on the substrate 114 efficiently and with high rate, without damaging the substrate or the devices on the substrate, and without contaminating the substrate. Further, it should be noted that the microwave emitted from the radial line slot antenna is blocked by the conductive structure and there is no possibility of such a microwave causes damaging in the substrate 114.
Meanwhile, the density of the plasma formed in the space 101B can reach the order of 1012/cm3 in such a plasma processing apparatus 110 that uses the radial line slot antenna 110. Thus, the shower plate 103 is exposed to a large amount of ions and electrons constituting the high-density plasma, and the ions and electrons thus formed cause heating. The thermal flux caused by such ions and electrons can reach the value of as much as 1–2 W/cm2. In view of the fact that the plasma processing apparatus 100 is frequently operated by maintaining the wall temperature of the processing chamber 101 to about 150° C. so as to suppress formation of deposits on the processing chamber 101, there is caused accumulation of heat in the shower plate 103 and the cover plate 102 formed of a dielectric material, as a result of heating of the processing chamber 101. As a result, there is formed a very large temperature distribution.
In order to reduce such accumulation of heat in the shower plate 103 and the cover plate 102, it is preferable that the radial line slot antenna 110 is made close contact to the cover plate 102 so as to remove the heat by using the antenna 110 as a heat sink. However, since the radiation plate 110C is fixed by screws to a central conductor of the coaxial waveguide 110A in the conventional radial line slot antenna 110, a space for the screw heads must be retained between the cover plate 102 and the radiation plate 110C, and, thus, it is difficult to adopt such a structure.
Additionally, in the conventional plasma processing apparatus 100, the radial line slot antenna 110 is subjected to substantial heating and the temperature thereof is increased even when not closely contact to the cover plate 102 due to the heat flux from the shower plate 103 and the cover plate 102. Moreover, when the radial line slot antenna 110 is made closely contact, the temperature rise of the antenna becomes still larger.
The conventional radial line slot antenna is not designed on the assumption of usage under such a high-temperature environment, and, therefore, if the temperature of the antenna rises in this way, a gap may be produced between the dielectric plate 110D provided as a wave retardation plate and the radiation plate 110C due to a difference in coefficients of thermal expansion. Thus, if such a gap is produced between the wave retardation plate 110D and the radiation plate 110C, the impedance which the microwave propagating inside the waver retardation plate senses is disturbed, and there occurs a problem such as an abnormal discharge, a formation of a reflection wave or a formation of a stationary wave within the antenna. If an abnormal discharge occurs, use of the antenna will become impossible thereafter.